In electronics equipment, insulated gate semiconductor devices, such as insulated gate bipolar transistors (IGBTs) and metal oxide semiconductor field effect transistors (MOSFETs), are widely used as switching elements for controlling power supply to loads such as motors. One example of such an insulated gate semiconductor device is a trench-gate MOSFET in which a gate electrode is embedded in a semiconductor layer.
For example, MOSFET cells (cell regions) connected in parallel are treated as one semiconductor device in a typical vertical MOSFET. In other words, a semiconductor device including a vertical MOSFET is formed by arranging MOSFET cells. Typical examples of the arrangement patterns of MOSFET include a cell pattern and a stripe pattern. In the cell pattern, MOSFET cells each including a square source region surrounded by a gate trench are arranged in a lattice pattern. In the stripe pattern, MOSFET cells are arranged in a comb pattern by disposing gate trenches between elongated, stripe-shaped source regions.
During the off state of the semiconductor device, an electric field tends to concentrate on an insulating film on the bottom of a trench in the trench MOSFET, on which, the reliability of the insulating film is low. To overcome such a problem, for example, Patent Document 1 proposes a technique of extending a depletion layer in a drift layer of first conductivity type on the bottom of the trench to provide, on the bottom of the trench, a protective diffusion layer of second conductivity type that can reduce an electric field to be applied to the insulating film on the bottom of the trench.
If a protective diffusion layer has a floating potential, electrical characteristics such as switching characteristics may become unstable, so it is desirable that the potential of the protective diffusion layer be grounded. For example, Patent Document 2 proposes a method for setting one of the MOSFET cells as a protective contact region and grounding the protective diffusion layer through the protective contact region in the cell pattern.